1. Field of the Invention
The present invention relates to a CN (Carrier to Noise) ratio measuring apparatus for measuring the CN ratio of signals sent along transmission paths such as a transmission path of a broadcast satellite or a communication satellite.
2. Description of the Prior Art
Conventionally, there are known three types of methods of measuring CN ratio (ratio of a carrier power to a noise power) for satellite broadcast signals. A first type is known as "an in-carrier noise equalizing method" which is disclosed in Japanese Patent Public-Disclosure No. 61-274495. The method is employed for detecting a noise power in a vertical blanking period of a TV video signal and calculates the ratio of the noise power to a separately measured carrier power to derive a CN ratio. A second type is known as "an SN ratio converting method" which demodulates a received signal and measures the SN ratio for the demodulated signal which is then converted to a CN ratio. A third type is a method using a spectrum analyzer. "New CATV Techniques" edited by Electric Telecommunication Technology Committee describes on pages 246-257 "CN Ratio Measuring Method for Signals Received through Satellite Broadcasting" using a spectrum analyzer. This method measures the electric power of noise immediately outside of the band of a carrier to be measured to derive a noise power of the carrier and calculate the ratio of the thus measured noise power to a separately measured power of the carrier.
Since the in-carrier noise equalizing method extracts noise in the vertical blanking period, it requires the use of a complicated demodulator. The second SN ratio converting method also requires a demodulator which must be stable over a wide band. Further, the third method using a spectrum analyzer not only requires an expensive spectrum analyzer but also is required to perform complicated calculations for deriving a CN ratio.